That's likely an issue with iMazing not properly handling apple's saved ui state mechanism. You could try checking if holding the option key reveals a "quit and close all windows" option in the app's eponymous menu item, or press and hold the Shift key while opening the app to forget previous UI state.

If you don't care for apps to remember their previous windows and ui state, you can disable this mechanism in the settings app: https://support.apple.com/en-ca/HT204005#appresume